Direct inhibitory action of EGTA-Ca complex on reverse-mode Na/Ca exchange in Myxicola giant axons

Giant axons from the marine annelid Myxicola infundibulum were internally dialyzed with solutions containing 22Na ions as tracers of Na efflux. In experiments performed in Li-substituted seawater, Na efflux that is dependent on external Ca ion concentration, [Ca2+]o, was measured using dialysis to maintain [Na+]i at 100 mM, which enhances the [Ca2+]o-dependent Na efflux component, (i.e., reverse-mode Na/Ca exchange). When dialysis fluid contained EGTA (1 mM) to buffer the internal Ca concentration, [Ca2+]i, to desired levels, Na efflux lost its normal sensitivity to external calcium. The inhibition was not simply due to the Ca-chelating action of EGTA to produce insufficient [Ca2+]i to activate Na/Ca exchange. The addition of EGTA inhibited Cao-dependent Na efflux even when a large enough excess of [Ca2+]i was present to saturate the EGTA and still produce elevated values of [Ca2+]i. Control experiments showed that these high values of [Ca2+]i resulted in normal Na/Ca exchange in the absence of EGTA. It is concluded that the presence of EGTA itself interferes with the manifestation of reverse-mode Na/Ca exchange in Myxicola giant axons.